New diorganotin(IV) derivatives of dipeptides: Synthesis and characteristic spectral studies

Article abstract of DOI:10.1016/j.saa.2008.01.006

Some new diorganotin(IV) derivatives of the formulae, R₂SnL, where R = Me, n-Bu, Ph, and n-Oct, and L is the dianion of histidinylalanine (H₂L-1) and histidinylleucine (H₂L-2) have been synthesized by the reaction of R₂SnCl₂ and the preformed sodium salt of the respective dipeptides. The bonding and coordination behaviour in these derivatives are discussed on the basis of FT-IR, multinuclear ¹H, ¹³C and ¹¹⁹Sn NMR and ¹¹⁹Sn Moessbauer spectroscopic studies. These investigations suggest that dipeptides in R₂SnL act as dianionic tridentate coordinating through the COO^-, NH₂ and N^-_peptide groups. The ¹¹⁹Sn Moessbauer studies, together with the NMR data, suggest a trigonal bipyramidal geometry around tin in R₂SnL with the alkyl/aryl groups and N_peptide in the equatorial positions, while a carboxylic oxygen and the amino nitrogen atom occupy the axial positions.

Full text of DOI:10.1016/j.saa.2008.01.006

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Spectrochimica Acta Part A 71 (2008) 529–536
New diorganotin(IV) derivatives of dipeptides: Synthesis and
characteristic spectral studies
Mala Natha,∗, Hitendra Singha, George Engb, Xueqing Songb
a Department of Chemistry, Indian Institute of Technology-Roorkee, Roorkee 247667, India
b Department of Chemistry and Physics, University of The District of Columbia, 4200 Connecticut Avenue, NW Washington, DC 20008, USA
Received 10 August 2007; accepted 4 January 2008
Abstract
Some new diorganotin(IV) derivatives of the formulae, R2SnL, where R = Me, n-Bu, Ph, and n-Oct, and L is the dianion of histidinylalanine
(H2L-1) and histidinylleucine (H2L-2) have been synthesized by the reaction of R2SnCl2 and the preformed sodium salt of the respective dipeptides.
The bonding and coordination behaviour in these derivatives are discussed on the basis of FT-IR, multinuclear 1H, 13C and 119Sn NMR and 119Sn
Mo¨ssbauer spectroscopic studies. These investigations suggest that dipeptides in R2SnL act as dianionic tridentate coordinating through the COO−,
NH2 and N−peptide groups. The 119Sn Mo¨ssbauer studies, together with the NMR data, suggest a trigonal bipyramidal geometry around tin in R2SnL
with the alkyl/aryl groups and Npeptide in the equatorial positions, while a carboxylic oxygen and the amino nitrogen atom occupy the axial positions.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Diorganotin(IV) complexes; Dipeptides; Multinuclear magnetic resonance; 119Sn Mo¨ssbauer
1. Introduction
potent anti-inflammatory activity, thus such compounds may
hold the potential to be placed in the class of nonsteroidal anti-
inflammatory drugs (NSAIDs). Therefore, considerable efforts
have been made to characterize model organotin compounds
of the ligands having hetero donor atoms (O, N and/or S)
[26(a–f)], and simultaneously several studies have been focused
on structure–activity correlations [27(a, b)] during the last two
decades.
In order to obtain a better insight into how the organotin
species behave inside biological system, it is necessary to study
their coordination behaviour with ligands that can occur in the
biological medium, and hence to formulate structure activity
correlations to devise new organotin derivatives with potential
anti-tumour and anti-inflammatory activities. In view of this,
here we report the synthesis and structural studies of some
diorganotin(IV) derivatives of dipeptides containing at least one
essential amino acid residue, viz., histidinylalanine (H2L-1) and
histidinylleucine (H2L-2).
Organotins, important environmental pollutants widely used
in agricultural and industrial applications, enter into the food
chain and accumulate in the environment [1]. However, organ-
otin(IV) compounds have emerged as potentially biologically
active compounds among non-platinum chemotherapeutic met-
allopharmaceuticals in the last two decades [2,3]. Among the
class of organotins, diorganotin compounds have attracted a
great deal of attention not only because of their wide range of
industrial and biological applications [4], but more because of
their place among non-platinum chemotherapeutic compounds
exhibiting good anti-tumour activity [2,3,5–15]. Due to the
potential biological activity of diorganotin compounds, dur-
ing the last decade considerable efforts have been directed to
study the interaction of diorganotin(IV) moiety with amino
acids [2,3,8,11,13,16–21] and peptides [2,3,8,12,17,20,22–25].
In view of this, the study of the interaction of diorganotin(IV)
moiety with the dipeptides containing at least one essential
amino acid residue is indispensable. Further, diorganotin(IV)
derivatives of dipeptides [17,22,23] have been found to exhibit
2. Experimental
2.1. Materials
∗
Allofthereactionswerecarriedoutunderananhydrousnitro-
gen atmosphere. Specially dried methanol (99.95%, v/v) was
Corresponding author. Tel.: +91 1332 285797; fax: +91 1332 273560.
E-mail address: malanfcy@iitr.ernet.in (M. Nath).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.01.006

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M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
TMS as the internal standard. 119Sn NMR spectra were recorded
on a Bruker DRX 500 (500 MHz FT NMR) spectrometer at
the Institute Instrumentation Centre, IIT, Roorkee, India, using
dried by refluxing it with Mg metal and then distilled, and other
solvents were dried and distilled before use. Dimethyltin(IV)
dichloride, di-n-butyltin(IV) oxide, diphenyltin(IV) dichloride
(E. Merck), di-n-octyltin(IV) oxide (Aldrich), histidinylalanine
(His-Ala), and histidinylleucine (His-Leu) (Sigma) were used
as-received.
DMSO-d6/CD3OD as solvent and TMS as the internal stan-
119
¨ ¨
Sn Mossbauer spectra were recorded on Mossbauer
dard.
spectrometer model MS-900 according to the procedure
reported previously [12], at the Department of Chemistry and
Physics, University of The District of Columbia, Washington,
D.C.
2.1.1. Synthesis of dimethyltin/diphenyltin(IV) derivatives
of dipeptides by sodium chloride method
The dipeptide (1.5 mmol) was dissolved in the minimum
amount (20 ml) of specially dried methanol under dry nitrogen
and added to sodium methoxide, prepared by reacting sodium
(0.081 g, 3.5 mmol) with dry methanol (25 ml). The resulting
mixture was first stirred at room temperature for half an hour and
then refluxed giving a clear solution of Na2L within half an hour.
Refluxingwascontinuedforanother4–6 hwithconstantstirring.
A hot methanol solution (20 ml) of dimethyltin/diphenyltin(IV)
dichloride (1.5 mmol)wasadded to thesolutionof the preformed
sodium salt of the dipeptide. The resulting solution was fur-
ther refluxed with constant stirring for another 14–16 h for the
diphenyltin(IV) derivatives whereas only stirring was carried out
at room temperature (30 2 ◦C) for the dimethyltin(IV) deriva-
tives, under dry nitrogen atmosphere. It was then centrifuged
and filtered in order to remove the sodium chloride formed.
The excess of solvent was removed under reduced pressure and
the solid product thus obtained was recrystallized from either
methanol–hexane or methanol–petroleum ether (bp 40–60 ◦C)
mixture (1:3, v/v).
3. Results and discussion
The reactions of R2SnCl2 (R = Me and Ph) with the sodium
salt of dipeptide ligand (formed according to Eq. (1)) in a 1:1
molar ratio led to the formation of the compounds according
to Eq. (2). Di-n-butyltin/di-n-octyltin(IV) oxides react with the
dipeptides in equimolar ratio in dry methanol to give the com-
pounds under azeotropic removal of water (Eq. (3):
H2L-1/H2L-2 + 2NaOMe1:2 in sp−. →dried MeOH
Na2(L-1)/Na2(L-2) + MeOH
(1)
(2)
(3)
R2SnCl2 + Na2(L-1)/Na2(L-2)1:1 in sp−. →dried MeOH
R2Sn(L-1)/R2Sn(L-2) + 2NaCl
Rꢀ SnO + H2L-1/H2L-21:1 in sp−. →dried MeOH
2
Rꢀ2Sn(L-1/L-2) + H2O
2.1.2. Synthesis of di-n-butyltin/di-n-octyltin(IV)
derivatives of dipeptides by the azeotropic removal of water
method
where R = Me and Ph; Rꢀ = n-Bu and n-Oct; H2L-1 = His-Ala
and H2L-2 = His-Leu.
The Me2Sn(IV), n-Bu2Sn(IV) and Ph2Sn(IV) derivatives
were synthesized within ∼18–20 h of refluxing. Whereas, the
reactions involving the synthesis of n-Oct2Sn(IV) derivatives
of H2L-1 and H2L-2 yielded a turbid solution after a prolonged
heating, and the solid was obtained from the filtrate after remov-
ing the unreacted peptide/organotin oxide. The resulting solids
were obtained in good yields (64–87%). All of the derivatives
are found to be stable towards air and moisture. Most of the
synthesized compounds are soluble in methanol, but sparingly
soluble in chloroform and other solvents upon heating. The ana-
lytical data of the derivatives, as presented in Table 1, suggest
that in every instance the resulting compounds crystallized with
1:1 stoichiometry regardless of the proportions of the organotin
moiety and dipeptide used (Scheme 1).
The compounds were prepared under anhydrous nitrogen
atmosphere by drop-wise addition of a dry, hot methanol solu-
tion of di-n-butyltin(IV) and di-n-octyltin(IV) oxide (1.5 mmol)
to a hot methanol solution of the dipeptide (1.5 mmol). The
reaction mixture obtained was refluxed with constant stirring
for at least 14–16 h with azeotropic removal of water. The
solution was filtered, and the excess of solvent was removed
under reduced pressure and allowed to cool. The solid product
thus obtained was recrystallized by either methanol–hexane or
methanol–petroleum ether (bp 40–60 ◦C) mixture (1:3, v/v).
2.2. Measurements
The melting points of the synthesized compounds were deter-
mined on a Toshniwal capillary melting point apparatus and
were uncorrected. Carbon, hydrogen and nitrogen analyses of
these compounds were carried out on a VarioEL, CHNS-rapid
elemental analyzer. The tin content in the synthesized com-
pounds was determined gravimetrically as SnO2 [12]. Infrared
and far-infrared spectra of the solid compounds were recorded
on a PerkinElmer 1600 series FT-IR spectrophotometer in the
range 4000–400 cm−1 from KBr discs and 600–200 cm−1 from
CsI discs. 1H and 13C NMR spectra were recorded on a Bruker
DRX 300 (300 MHz FT NMR) spectrometer at the Central Drug
Research Institute, Luknow, India, using CD3OD as solvent and
3.1. Infrared spectral studies
The characteristic infrared absorption frequencies (in cm−1
)
and their assignments for the free dipeptides and their diorgan-
otin(IV) derivatives are presented in Table 2.
3.1.1. Coordination by amino group
The infrared –NH2 stretching frequencies were used to
distinguish coordinated from non-coordinated amino groups
of the dipeptides/amino acids. The position and intensity of

M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
531
Scheme 1.
ν(N H) bands are influenced by hydrogen bonding, and by
coordination of the nitrogen to tin [2,12]. In all of the stud-
ied diorganotin(IV) derivatives of dipeptides, very intense
absorption bands in the range 2920–3397 cm−1 due to the
ν(N H)amino, undergo a substantial lowering in comparison to
the non-coordinated dipeptides H2L-1 (3464 cm−1) and H2L-2
(3421–3078 cm−1), indicating coordination by the amino group
to the central tin atom. Similar results have been reported for
R3SnAA (AA = amino acid anion) [2,13,16,18,19,21,28] and
R2SnL (H2L = dipeptide) [12,17,22,23,29,30]. The appearance
of a new band of medium intensity in the region ∼409–500 cm−1
in all of the derivatives studied, which may be assigned to
ν(Sn←N), further confirms the coordination of the amino nitro-
gen to the organotin(IV) moiety. Further, the ν(NH2) absorption
bands are broad, suggesting the presence of inter- and/or intra-
molecular hydrogen bonding [2,12,17,22,23,28].
3.1.2. Coordination by carboxylate group
Infrared O C O stretching frequencies have been utilized to
distinguish coordinated from non-coordinated carboxyl groups,
and also, to identify the nature of bonding of carboxylic
group, viz., monodentate or bridging. The carboxylate groups
in the organotin(IV) derivatives generally adopt a bridged struc-
ture in the solid state unless the organic substituents at tin
are bulky or unless the carboxylate group is branched at the
␣-carbon [29]. The infrared absorption spectra of all the diorgan-
otin(IV) derivatives of the dipeptides indicate that νas(O C O)
values shown by these amino-coordinated complexes get
shifted to higher frequencies (1585–1635 cm−1) in compari-
son to those of H2L-1 (1566 cm−1) and H2L-2 (1565 cm−1).
Whereas, the corresponding νs(O C O) absorption frequen-
cies (1381–1438 cm−1) slightly moves either to lower or
higher frequencies than in the non-coordinated dipeptides
(H2L-1 = 1400 cm−1, H2L-2 = 1390 cm−1). The magnitude of
the (νas–νs)(O C O) (ꢀν) separation, which has been found
to be useful in identifying structural features [2], is larger
in the amino-coordinated diorganotin(IV) derivatives of H2L-
1 (ꢀν = 226–241 cm−1) (except Ph2Sn(L-1)) and H2L-2
(ꢀν = 174–193 cm−1) than in the non-coordinated dipeptides,
H2L-1 ꢀν = 166 cm−1 and H2L-2 ꢀν = 175 cm−1 (Table 2).
Further, the magnitude of ꢀν for all these studied deriva-
tives of H2L-1 have been found comparable to those obtained
for R3SnAA (AAH = amino acid) [2,13,16,18,19,21,28] and
R2SnL (H2L = dipeptide) [12,17,22,23], indicating that the car-

532
M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
Table 2
Characteristic IR frequenciesa (in cm−1) of diorganotin(IV) derivatives of H2L-1 (His-Ala) and H2L-2 (His-Leu)
Ligand/complex
ν(NH) amino/ν(NH) pep.
ν(CO) amide
νas(OCO)
1566 vs
1594 vs
1607 sh*
1635 m
νs(OCO)
1400 m
1438 m
1381 m
1394 m
ꢀν
νas(Sn–C);
νs(Sn–C)
ν(Sn–O)
–
ν(Sn–N)/ν(Sn←N)
H2L-1
(ligand)
3464 sbr
1677 sh
1634 s
166
156
226
241
–
–
Ph2Sn(L-1)
Me2Sn(L-1)
n-Bu2Sn(L-1)
3383 s
3271 s
1633 s
1632 vs
1681 s
270 m
228 m
587 m
566 m
566 m
486 m; 447 m
417 w
3367 s
3175 sh
616 w
494 sh
3207 s
3141 s
3033 s
2954 s
625 m
597 m
499 w
414 m
n-Oct2Sn(L-1)
3035 m
2954 s
2920 s
1680 s
1635 w
1565 vs
1394 m
1390 vs
241
175
625 m
602 m
560 m
–
500 m
409 w
H2L-2
(ligand)
3421 mbr
3213 s
1683 vs
–
–
3078 s
2952 vs
Ph2Sn(L-2)
Me2Sn(L-2)
n-Bu2Sn(L-2)
3397 sbr
3343 s
1644 s
1585 vs
1587 s
1399 s
1394 s
1394 s
1393 m
186
193
174
175
206 m
228 m
247 m
565 m
582 m
564 m
560 m
448 m
3395 sbr
3348 sh
3239 s
1643 vs
1678 sh
664 w
613 w
464 m
422 w
3210 s
3084 s
2957 s
1679 sh
1613 s
1568 sh*
667 m
620 w
474 wsh
414 m
n-Oct2Sn(L-2)
3212 m
3083 m
2922 s
1680 s
1613 sh
1589 m
666 m
600 w
415 m
470 w
a
Intensity of characteristic bands as: vs, very strong; s, strong; m, medium; w, weak; sh, shoulder; br, broad; *merge with ν(CO) amide.
boxylate group acts as a monodentate ligand. The magnitude
of the (νas–νs) (O C O) (ꢀν) separation in the diorgan-
otin(IV) derivatives of H2L-2 is <200 cm−1, indicating that the
carboxylate group is bidentate and bridging. Similar results
are also reported for R2SnGlyGly (R = Me and n-Bu), n-
Bu2SnGlyGly·H2O and Me2SnGlyAla [30]. Furthermore, the
disappearance of a broad band in the spectra of all of the
derivatives in the region 2750–2600 cm−1, which was present
in both the dipeptides as a weak intensity band (due to
ν(O H)carboxyl), suggests the deprotonation of the COOH group
upon complexation [2]. The appearance of a medium inten-
sity band in the IR spectra of all the compounds in the region
587–560 cm−1, which may be assigned to ν(Sn O), further
supports the bonding of (O C O) group to the tin atom
[2,12,16,17,21–23].
viz., amide I [essentially ν(C O)] and amide II [δ(N H) coupled
with ν(C N)], give the crucial information on the occurrence
of metal coordination by the basic atoms of the amide group
[12,31]. The intense bands of the amide I observed at 1677,
1634 cm−1 and 1683 cm−1 in H2L-1 and H2L-2, respectively,
undergo a slight shift to a lower frequency (1681–1613 cm−1) in
the IR spectra of the diorganotin(IV) derivatives of H2L-1 and
H2L-2 (except Bu2Sn(L-1) and Oct2Sn(L-1)) upon complex-
ation. This is probably due to the involvement of the peptide
nitrogen (because of the deprotonation that has taken place) in
bonding with tin, which lowers the bond order of the (C O)amide
group due to the resonance stabilization. The possibility of
the involvement of the (C O)amide group in the intermolecular
hydrogen bonding can not be excluded. Further, amide II band
observed at 1557 cm−1 in H2L-1 and at 1535 cm−1 in H2L-2
gets shifted to lower frequency upon complexation in all of the
diorganotin(IV) derivatives of dipeptides with respect to non-
coordinated dipeptides, which suggests that the amide nitrogen
is the third coordinating site due to the deprotonation of the
amide nitrogen. The appearance of a pair of bands of medium
intensity in the region (450 40) cm−1 which may be assigned
3.1.3. Coordination by peptide group
In the derivatives studied, apart from the carboxylic oxygen
and amino nitrogen as potential coordinating sites to the tin
atom, the amide group also exhibits strong tendency to coor-
dinate with the organotin(IV) moiety. Two characteristic bands,

M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
533
Table 3
1H NMR spectral data of the diorganotin(IV) derivatives of H2L-1 (His-Ala) and H2L-2 (His-Leu)
Complex number
H2L-1
Complex/ligand (solvent)
δ (ppm)a
His-Ala (CD3OD + DMSO-d6 at 500 MHz)
H-2: 4.28 (q, 7.0, 7.5, 7.0 Hz, 1H); H-3: 1.45 (d, 7.0Hz, 3H); H-5: 4.09 (t, 5.2Hz, 1H); H-6:
3.27, 3.25 (dd, 5.5Hz, 2H) {10.0 Hz} ; H-8: 7.86 (s, 1H); H-9: 7.07 (s, 1H)
c
1
2
3
Ph2Sn(L-1) (CD3OD at 300 MHz)
Me2Sn(L-1) (CD3OD at 300 MHz)
n-Bu2Sn(L-1) (CD3OD at 300 MHz)
H-2: 4.19 (q, 7.2 Hz, 1H); H-3: 1.34 (d, 7.2 Hz, 3H); H-5: 3.57 (t, 6.0 Hz, 1H); H-6: 2.96, 2.93
(dd, 5.7, 6.6 Hz, 2H) [38.7/37.8 Hz]b; H-8: 7.54 (s, 1H); H-9: 6.82 (s, 1H); H-␣: 7.77 (d,
7.2 Hz, 4H) [61.5/59.1 Hz]b; H-␤ + H-␥: 7.45 (m, 6H)
H-2: 4.17 (q, 6.9, 6.6, 6.9 Hz, 1H); H-3: 1.28 (d, 6.9 Hz, 3H); H-5: 3.84, 3.82 (dd, 3.9, 4.2 Hz,
1H); H-6: 3.21, 2.12 (dd, 3.6, 7.5 Hz, 2H) [59.1/55.5 Hz]b; H-8: 7.77 (s, 1H); H-9: 7.02 (s,
1H); Sn–CH3: 0.75, 0.72 (s, 6H) [79.2/78.9 Hz]b
H-2: 4.13 (q, 6.9 Hz, 1H) [20.7 Hz]; H-3: 1.24 (d, 6.9 Hz, 3H); H-5: 3.75 (t, 5.0 Hz, 1H); H-6:
c
3.15, 3.09 (dd, 6.0, 4.2 Hz, 2H), {18.0 Hz} [48.9/47.1]b; H-8: 7.66 (s, 1H); H-9: 6.92 (s, 1H);
H-␣: 1.35 (t, 7.2 Hz, 4H); H-␤: 1.53 (t, 6.3 Hz, 4H); H-␥: 1.44, 1.42 (m, 4H); H-␦: 0.91 (t,
6.3 Hz, 6H)
4
5
6
n-Oct2Sn(L-1) (CD3OD at 300 MHz)
Ph2Sn(L-2) (CD3OD at 300 MHz)
Me2Sn(L-2) (CD3OD at 300 MHz)
H-2: 4.23 (q, 7.2 Hz) and 4.13 (q, 6.9 Hz, 1H); H-3: 1.39 (d, 7.2 Hz, 3H); H-5: 3.98 (t, 5.7 Hz)
and 3.74 (t, 2.2 Hz, 1H); H-6: 3.18 (d, 5.7 Hz), 3.12 (d, 3.6 Hz) and 3.31 (s, 1H); H-8: 7.80,
7.65 (s, 1H); H-9: 7.01, 6.92 (s, 1H); H-octyl: 1.30, 0.93 (m)d
H-2: 4.32, 4.28 (dd, 3.3, 3.9 Hz, 1H); H-3 + H-4: 1.68–1.56 (m, 3H); H-5a: 0.92 (d, 5.4 Hz,
3H); H-5b: 0.91 (d, 4.8 Hz, 3H); H-7: 3.59 (t, 1.9 Hz, 1H); H-8: 2.98 (d, 4.5 Hz, 2H); H-10:
7.54 (s, 1H); H-11: 6.82 (s, 1H); H-␣: 7.78 (d, 6.3 Hz, 4H); H-␤ + H-␥: 7.74 (m, 6H)
H-2: 4.21 (t, 4.8, 6.0 Hz, 1H), 4.30 (d, 3.6 Hz) and 4.27 (d, 3.9 Hz, 1H); H-3 + H-4: 1.68–1.53
(m, 3H) (1.42–1.38 (m, 3H)); H-5a + H-5b: 0.91 (t, 5.2 Hz, 6H) (0.84 (d, 6.0 Hz, 6H); H-7:
3.59 (t, 5.7 Hz, 1H) (3.79, 3.77 (dd, 4.5, 4.5 Hz, 1H)); H-8: 2.97 (d, 5.4 Hz, 2H) (3.11 (t, 4.5,
6.5 Hz, 2H)); H-10: 7.56 (s, 1H); H-11: 6.84 (s, 1H); Sn–CH3: 0.74, 0.69 (s, 6H) (0.56, 6H)
7
n-Bu2Sn(L-2) (CD3OD at 300 MHz)
n-Oct2Sn(L-2) (CD3OD at 300 MHz)
H-2: 4.21 (t, 5.7 Hz, 1H) [37.5 Hz]; H-3 + H-4: 1.50–1.64 (m, 3H); H-5a + H-5b: 0.83 (d,
6.3 Hz, 6H); H-7: 3.74 (t, 4.8 Hz, 1H); H-8: 3.16, 3.21 (dd, 5.7, 5.7 Hz, 2H), 3.05, 3.00 (dd,
3.9, 4.2 Hz, 2H) [15.0 Hz]; H-10: 7.62 (s, 1H); H-11: 6.91 (s, 1H); H-␣: 0.96 (d, 7.2 Hz, 4H);
H-␤ + H-␥: 1.30–1.47 (m, 8H); H-␦: 0.90 (t, 7.2 Hz, 6H)
8
H-2: 4.21 (t, 5.5 Hz, 1H); H-3 + H-4: 1.50–1.63 (m, 3H); H-5a + H-5b: 0.84 (t, 6.6 Hz, 6H);
H-7: 3.74 (t, 4.9 Hz, 1H), 3.30 (t, 1.3 Hz, 1H); H-8: 3.19, 3.15 (dd, 6.0, 6.0 Hz, 2H), 3.05, 3.01
(dd, 4.2, 4.0 Hz, 2H); H-10: 7.61 (7.78) (s, 1H); H-11: 6.91 (7.0) (s, 1H); H-␣ to H-g:
1.50–1.15 (m, 28H); H-h; 0.89 (t, 3.3 Hz, 6H)
a
Homonuclear proton–proton coupling multiplet abbreviations given in parentheses: s, singlet; d, doublet; t, triplet; q, quartet; dd, doubletdoublet; m, complex
multiplet.
b
2J/nJ(1H−117/119Sn) coupling constants for the alkyl/phenyl groups are given between square brackets; small peak in parenthesis.
c
Geminal coupling.
d
Overlapping multiplets
.
to ν(Sn N) and ν(Sn←N), further confirms the coordination
of the amino nitrogen as well as the peptide nitrogen to the
diorganotin(IV) moiety [2,12,17,22,23].
The νas(Sn C) and νs(Sn C) bands in all the dialkyltin(IV)
derivatives are observed in the range 597–625 cm−1 for H2L-1
and 613–667 cm−1 for H2L-2, suggesting the existence of a bent
C Sn C moiety [21,22], whereas in the diphenyltin(IV) deriva-
tives, the corresponding νas(Sn C) and νs(Sn C) are observed
at 270 2 and 228 2 cm−1, respectively [21,22].
the derivatives studied, the CO(OH) resonance of the ligands
(δ 12.0–13.0 ppm) is absent which suggests the replacement
of the carboxylic proton by the organotin(IV) moiety. In all of
the diorganotin(IV) derivatives, the –NH2 resonances observed
either as a broad weak signal or in conjugation with phenyl
protons attached to the tin, are shifted towards low field in the
ranges δ 7.45–7.78 ppm, when compared to those of the ligands
(δ 5.0–8.0 ppm) [32]. This is probably due to the coordination
of the amino group to the organotin(IV) moiety. As reported
previously [2,13,17,19,21,22], upon complexation the magnet-
ically non-equivalent alkyl protons of the ligands undergo the
diamagnetic shielding due to the conformation adopted by the
ligand molecules. The resonances due to the tin-alkyl protons in
the studied Me2Sn(IV), n-Bu2Sn(IV) and n-Oct2Sn(IV) deriva-
tives are observed in the region δ 0.69–0.75, 0.90–1.53 and
0.89–1.50 ppm, respectively, whereas the tin-phenyl protons in
the diphenyltin(IV) derivatives are observed in the regions δ
3.2. Solution NMR spectral studies
3.2.1. 1H NMR spectral analysis
The characteristic resonance peaks in the 1H NMR spectra of
thestudiedderivatives, recordedinmethanol-d4, arepresentedin
Table 3. The 1H NMR spectral data of H2L-1 are also included
1
in Table 3 for comparison. In the H NMR spectra of all of

534
M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
Table 4
13C NMR spectral data of the diorganotin(IV) derivatives of H2L-1 (His-Ala) and H2L-2 (His-Leu)
Complex number
H2L-1
Complex/ligand (solvent)
δ (ppm)
His-Ala (CD3OD + DMSO-d6 at 500 MHz)
C-1: 178.0; C-2: 48.4; C-3:16.0; C-4: 167.6; C-5: 55.5; C-6: 36.1; C-7: 111.0; C-8:134.6; C-9:
134.0
1
2
3
4
Ph2Sn(L-1) (CD3OD at 300 MHz)
Me2Sn(L-1) (CD3OD at 300 MHz)
Bu2Sn(L-1) (CD3OD at 300 MHz)
Oct2Sn(L-1) (CD3OD at 300 MHz)
C-1: 180.2; C-2: 51.8; C-3: 18.9; C-4: 175.6; C-5: 55.6; C-6: 32.9; C-7: 121.7; C-8: 136.7;
C-9: 132.1; C-␣: 141.2; C-␤: 137.6 [37.4 Hz]; C-␥: 129.7 [60.4 Hz]; C-␦: 130.5
C-1: 181.2; C-2: 53.1; C-3: 20.0; C-4: 174.9; C-5: 55.9; C-6: 31.1; C-7: 117.8; C-8: 136.6;
C-9: 134.9; C-␣: 0.06, 0.00
C-1: 181.3; C-2: 53.3; C-3: 19.7; C-4: 175.2; C-5: 56.2; C-6: 31.2; C-7: 117.3; C-8: 136.7;
C-9: 135.4; C-␣: 21.0, 20.7 [58.0 Hz]a; C-␤: 28.2 [38.0 Hz]a; C-␥: 27.7, 27.5; C-␦: 13.9
C-1: 181.5; C-2: 52.0 (51.7); C-3: 18.6, 19.7; C-4: 170.1; C-5: 56.2; C-6: 33.0; C-7: 121.1,
117.3; C-8: 137.1; C-9: 136.7; C-␣: 26.1; C-␤: 34.8, 34.5; C-␥: 31.3; C-␦: 30.3; C-e: 30.2;
C-f: 23.7; C-g: 21.4, 21.1; C-h: 14.4
5
6
Ph2Sn(L-2) (CD3OD at 300 MHz)
Me2Sn(L-2) (CD3OD at 300 MHz)
C-1: 180.2; C-2 + C-7: 54.8; C-3: 42.9 (35.1); C-4: 23.8; C-5a + H-b: 22.0; C-6: 175.2; C-8:
26.2 (32.7); C-9: 136.6; C-10 + C-␦: 130.6; C-11 + C-␥: 129.7; C-␣:153.2; C-␤:137.6
C-1: 177.4; C-2: 52.7 (53.2); C-3: 40.0 (40.9); C-4: 23.2 (22.3); C-5a: 20.9 (21.2) C-5b: 19.1
(20.1); C-6: 172.9; C-7: 51.9 (53.1); C-8: 29.9 (28.1); C-9: 119.1 (114.3); C-10 + C-11: 133.7;
C-␣: −3.5
7
8
a
Bu2Sn(L-2) (CD3OD at 300 MHz)
Oct2Sn(L-2) (CD3OD at 300 MHz)
C-1: 181.1; C-2: 56.6; C-3: 44.4; C-4: 25.4, 24.1; C-5a + C-5b: 21.2, 20.5; C-6: 175.0; C-7:
56.2; C-8: 31.1; C-9: 117.1; C-10 + C-11: 136.7 (136.2); C-␣: 23.3; C-␤: 28.4 [38.0 Hz]; C-␥:
27.9 (27.7); C-␦: 14.1
C-1: 181.1; C-2: 56.6; C-3: 44.4; C-4: 24.2; C-5a + C-5b: 21.0, 21.5; C-6: 175.0; C-7: 56.3;
C-8: 31.1; C-9: 117.1; C-10: 136.7; C-11: 124.4; C-␣: 26.3; C-␤: 35.0, 34.8; C-␥: 33.2 [88.5];
C-␦: 30.5, 30.4; C-e: 25.4; C-f: 23.9; C-g: 23.3; C-h: 14.6
nJ(13C−119Sn) coupling constants are given between square brackets; weak signals are in parenthesis
.
7.45–7.78 ppm [13,17,19,21,22]. The resonances due to all the
magnetically non-equivalent protons in the complexes have been
successfully identified, and the total numbers of protons calcu-
lated from the integration curve are in agreement with those
calculated from the proposed molecular formula.
• The carbons of phenyl (δ 129.70–153.20 ppm) and alkyl (δ
−3.50 to 35.00 ppm) groups attached to tin are observed
at positions comparable with other, similar compounds
[2,12,16,17,22,23].
The resonances of the COpeptide also get substantial upfield
shift (δ 170.10–175.60 ppm) in comparison to the ligand H2L-1
(δ 167.60 ppm) due to the presence of the inter-/intra-molecular
hydrogen bonding to the some extent. The observed downfield
shifts in the magnetically non-equivalent alkyl carbons of the
dipeptides upon complexation are due to: (i) the coordination
of the COO−, Namino and Np−eptide to the dialky/diphenyltin(IV)
moiety, (ii) the resonance in the –CONH– group, and (iii) the
inter-/intra-molecular hydrogen bonding. The above observa-
tions are consistent with the infrared and 1H NMR spectral data.
Moreover, in Me2Sn(IV) derivatives, doublets of some signals
have been observed due to the presence of stereoisomers.
3.2.2. 13C NMR spectral analysis
The characteristic resonance peaks in the 13C NMR spectra
of all of the studied derivatives, recorded in deuterodimethyl-
sulfoxide/deuteromethanol, are presented in Table 4. The 13C
NMR spectral data of H2L-1 are also included in Table 4. The
13C NMR spectrum of the H2L-2 could not be recorded because
of its extremely low solubility in DMSO-d6/CDCl3/CD3OD.
The spectra of the organotin(IV) derivatives of H2L-1 and
H2L-2 are consistent with the following observations:
• The resonances of the carboxylic carbon (i.e. C-1) in
all of the studied derivatives are observed at larger δ (δ
181.50–180.20 ppm) than in the ligand H2L-1 (δ 178.00 ppm)
except in the compound 6, suggesting the coordination of
the dipeptides, through the carboxylic oxygen to the organ-
otin(IV) moiety [2,12,22].
• Various carbons of the ligand; especially C-2, undergo a
downfield shift upon complexation as compared with that of
H2L-1, indicating the strong interactions of the O C O with
tin.
3.2.3. 119Sn NMR spectral analysis
The characteristic resonance peaks in the 119Sn NMR spectra
of all of the studied derivatives except Ph2Sn(L-2), recorded in
deuteromethanol, are presented in Table 5. The 119Sn chemi-
cal shifts of Me2Sn(L-1/L-2), n-Bu2Sn(L-1/L-2), Ph2Sn(L-1),
and Oct2Sn(L-1/L-2) are observed in the range 100.00 to
−188.20 ppm which are characteristic of the five-coordinated
dialkyl- and diphenyl-tin derivatives [2,12,16,17,22]. Further,

M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
535
Table 5
119Sn NMR spectral data of the diorganotin(IV) derivatives of H2L-1 (His-Ala)
and H2L-2 (His-Leu)
Complex number
Complex
δ (ppm)
1
2
3
4
6
7
8
Ph2Sn(L-1)
Me2Sn(L-1)
Bu2Sn(L-1)
Oct2Sn(L-1)
Me2Sn(L-2)
Bu2Sn(L-2)
Oct2Sn(L-2)
−188.2
−120.0, −142.0
−145.5
100.0
−170.50
−142.60
−143.15
119Sn NMR spectra of Me2Sn(L-1) gave additional peak at
−141.86 ppm indicating the presence of stereoisomers.
3.3. 119Sn Mo¨ssbauer spectral studies
The 119Sn Mossbauerparametershavebeenutilized asadiag-
¨
Fig. 1. Proposed structure of diorganotin(IV) hitidinylalaninates/histidinylleu-
cinates.
nostic tool for proposing the structure that a particular compound
can adopt in the solid state. These parameters point out whether,
the coordination of the amino group nitrogen atom, bonding of
the N−peptide and the carboxylic oxygen to tin lead to chelation or
electron density as well as the large asymmetry of the electron
distribution around the tin atom [29,30]. This is probably due
to stronger bonding of the ligands to the dialkyltin(IV) than to
the diphenyltin(IV) moiety, and partly due to the strain devel-
oped in the ligand. It has been reported that the replacement
of an alkyl group by a phenyl group lowers the isomer shift
in the organotin(IV) derivatives of the dipeptides/amino acids
[12,16,17,22,30,33]. Similar trend is also observed in I.S. and
Q.S. values of the diphenyltin(IV) complexes studied (Table 6).
I.S. and Q.S. values observed in R2Sn(IV) derivatives
of the peptides are slightly lower than those of previ-
ously reported complexes [12,17,22,30]. The spectroscopic and
crystallographic studies reported a distorted trigonal bipyra-
midal configuration for R2Sn(L) (where H2L = dipeptide),
where the organic groups of the organotin(IV) moiety and
peptide nitrogen are lying in equatorial position, and the
amino nitrogen and carboxylic oxygen atoms are axial
[2,12,17,22,30]. Thus, the tin atom configuration as shown
in Fig. 1 can be again proposed for R2Sn(L-1)/(L-2), which
would then be [histidinylalaninato/histidinylleucinato-O,N,N-
(2-)diorganotin(IV)], mainlyonthebasisoftheabovementioned
similarity between the observed and reported Q.S. values
[2,12,17,22,30], taking also into account the symmetry as
polymerization. The 119Sn Mossbauer spectral data of all of the
¨
studied compounds are presented in Table 6. The narrowness of
the linewidths, τ, except the diphenyltin(IV) derivatives, implies
the general occurrence of single tin sites in each compound, as
well as of multiple coordination sites with corresponding envi-
ronment [30]. The experimental nuclear quadrupole splitting
(Q.S.) values of the solid-state R2Sn(IV) derivatives (R = Me,
n-Bu, Ph and n-Oct), presented in Table 6, describe two classes
of compounds:
(i) Those having
a doublet centered in the region
0.40–0.60 mm s−1; quadrupole splitting (Q.S.) in the
region 1.00–1.54 mm s−1 for Ph2Sn(L-1) and Ph2Sn(L-2).
(ii) Those having
a doublet centered in the region
0.90–1.16 mm s−1; quadrupole splitting (Q.S.) in the
region 1.97–3.03 mm s−1 for R2Sn(L-1) and R2Sn(L-2)
(R = Me, n-Bu and n-Oct).
These observations indicate that on going from class (i)
to class (ii) compounds, both I.S. (isomer shift) and Q.S.
(quadrupole splitting) values increase due to an increase in s-
Table 6
119Sn Mo¨ssbauer data (80 K) of the diorganotin(IV) derivatives of H2L-1 (His-Ala) and H2L-2 (His-Leu)
Complex numbera
Complex
Q.S. (mm s−1
)
I.S. (mm s−1
)
ρ (Q.S/I.S.)
τ1(L)
τ2(R)
1
2
3
4
5
6
7
8
Ph2Sn(L-1)
1.00
2.90
3.03
2.12
1.54
2.17
1.97
2.33
0.40
1.16
1.03
1.00
0.60
0.90
0.95
1.03
2.50
2.50
3.00
2.12
2.56
2.41
2.07
2.26
1.20
1.15
3.50
1.03
1.28
1.02
2.65
1.12
2.00
1.26
4.00
1.13
1.81
1.13
2.68
1.46
Me2Sn(L-1)
n-Bu2Sn(L-1)
n-Oct2Sn(L-1)
Ph2Sn(L-2)
Me2Sn(L-2)
n-Bu2Sn(L-2)
n-Oct2Sn(L-2)
a
Q.S., quadrupole splitting; I.S., isomeric shift relative to BaSnO3 and tin foil (splitting: 2.52 mm s−1); τ1(L): half line-width left doublet component; τ2(R): half
line-width right doublet component (mm s−1).

536
M. Nath et al. / Spectrochimica Acta Part A 71 (2008) 529–536
well as the chelation constraints of the coordinated ligands
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tin nucleus, so that {N} would not be very much different
tbe
from {R} [34,35]. The lower values of I.S. and Q.S. in all
the diorganotin(IV) derivatives of dipeptides may be due to the
almost symmetrical distribution of charge in the plane contain-
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¨
from IR data). Further, the Mossbauer data (Table 6) indicate
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Thanks are due to the DST, New Delhi (India) for sanc-
tioning the research project to Prof. Mala Nath (grant no.
SP/S1/F07/2000) sponsored by DST, New Delhi, India. H. Singh
is grateful to DST, New Delhi, India, for awarding JRF under the
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